The vapor compression distillation system of the present invention employs a plate-in-frame type heat exchanger as disclosed in my copending patent application Ser. No. 08/320,233, filed Oct. 11, 1994, and which is constructed and arranged to cause a majority of the liquid on the condensing surface to occupy a minority of the condensing surface area to thereby improve the overall heat transfer between the boiling feed and condensing vapors. The plate-in-frame type heat exchanger disclosed in the aforementioned patent application is an improvement on the plate-in-frame type heat exchanger disclosed in U.S. Pat. No. 4,671,856. The vapor compression distillation system of the present invention is a further improvement over the system disclosed in the above-noted patent in that the compressed vapor is de-superheated as it enters the condensing chambers of the boiler/condenser assembly, and the heat extracted from the compressed vapor is transferred into the boiler in order to prevent heat build-up in the condenser from decreasing the energy efficiency of the process.
The superheating of compressed vapor has long been recognized as an undesirable but unavoidable consequence of compressing the vapors in a fixed displacement roots type blower. In relation to vapor compression between the boiler and condenser, superheat is particularly undesirable because the lower heat transfer coefficient of superheated steam requires a higher differential temperature for a given condensation rate. The higher differential temperature implies a lower efficiency for the process. Attempts have been made to correct the problem but they usually involve additional equipment and do not manage to utilize all of the heat extracted from the vapor in the process of de-superheating it.
A further improvement over the system disclosed in the above-noted patent is also provided by operating the system under sufficient vacuum to match the boiler temperature with the temperature of the incoming feed fluid, to thereby eliminate the requirement for, and heat loss implicit in, a waste heat recovery system. The elimination of a waste heat recovery system is advantageous in that it eliminates capital and operating costs for heat exchangers, pumps, and flow control systems, and the overall thermal efficiency of the process is improved since a heat exchanger based waste heat recovery system will be limited in its ability to recover heat by the approach temperature of the heat exchanger.